In USA, Vuthy Sim and Gary Majdell ( both on Amazon) have flag suits in various cuts.

Also, check out skinzweardotcom.

And swimoutletdotcom has flag suits under polo section.

Wearing the flag
 

Just to keep you all up-to-date.
I have now been wearing my Neptune Scepter Union Flag swim briefs for my early morning lane swimming at my local pool in North London. They are great to swim in with the very smooth material and just as comfortable as my speedos and Arena briefs.
I have had a number of comments - all favourable! (Even from an 80 year old lady!).

Incidently, I did a spot count on who was wearing what in the "fast lane" where I was swimming. Results:
Speedo briefs 3
Square trunks 1
Speedo jammers 1
Shorts 2
(and the females were all wearing Speedo one piece - no bikinis)

So speedo type briefs win!
and guys have briefer swimwear than the ladies!